W3 Total Cache's approach to conditional loading is rather elegant.

gistfile1.php

<?php

/**
 * Skip caching for some pages
 */
switch (true) {
    case defined('DONOTCACHEPAGE'):
    case defined('DOING_AJAX'):
    case defined('DOING_CRON'):
    case defined('APP_REQUEST'):
    case defined('XMLRPC_REQUEST'):
    case defined('WP_ADMIN'):
    case (defined('SHORTINIT') && SHORTINIT):
        return;
}